Atomically-aligned growth of a crystalline film atop a substrate—heteroepitaxy—has largely relied on vacuum methods such as molecular-beam epitaxy (MBE), atomic-layer epitaxy (ALE), and metalorganic vapour phase epitaxy (MOCVD) (1,2). Through these methods, theoretical predictions have been tested and refined that describe conditions under which crystalline coherence can be preserved even in the presence of a mismatch in the native lattices (strained-layer epitaxy). The result is a vast body of theory, knowledge, and practice regarding vapour-phase epitaxy. Since interfacial defects can be rendered rare at suitably-designed heterointerfaces, highly efficient luminescent materials have been created that have enabled efficient electrically-injected lasers and light-emitting diodes both for fiber-optic communications and high-efficiency lighting (3-5).
The past two decades have seen the rapid rise of soft condensed matter, often in the form of solution-processed semiconductors based on organic molecules, polymers, and colloidal nanoparticles (plates, wires, and dots) (6-10). In this vein, and with astonishing rapidity, bulk organohalide semiconductor perovskites exhibiting large and perfect crystalline domains have improved in size, properties, and performance. These remarkable materials have enabled the ascent of the perovskite solar cell (11-13).